1. Field of the Invention
The present invention relates to a method of manufacturing a spacer for supporting a pair of substrates, a method of manufacturing an image forming apparatus using the spacer, and an apparatus for manufacturing the spacer.
2. Description of the Related Art
Conventionally, two types of devices, namely thermionic and cold cathodes, are known as electron-emitting devices. Known examples of the cold cathodes are surface-conduction emission type electron-emitting devices, field emission type electron-emitting devices (to be referred to as FE type electron-emitting devices hereinafter), and metal/insulator/metal type electron-emitting devices (to be referred to as MIM type electron-emitting devices hereinafter).
A known example of the surface-conduction emission type electron-emitting devices is described in, e.g., M. I. Elinson, “Radio Eng. Electron Phys., 10, 1290 (1965) and other examples will be described later.
The surface-conduction emission type electron-emitting device utilizes the phenomenon that electrons are emitted by a small-area thin film formed on a substrate by flowing a current parallel through the film surface. The surface-conduction emission type electron-emitting device includes electron-emitting devices using an Au thin film [G. Dittmer, “Thin Solid Films”, 9,317 (1972)], an In2O3/SnO2 thin film [M. Hartwell and C. G. Fonstad, “IEEE Trans. ED Conf.”, 519 (1975)], a carbon thin film [Hisashi Araki et al., “Vacuum”, Vol. 26, No. 1, p. 22 (1983)], and the like, in addition to an SnO2 thin film according to Elinson mentioned above.
FIG. 20 is a plan view showing the device by M. Hartwell et al. described above as a typical example of the device structures of these surface-conduction emission type electron-emitting devices. Referring to FIG. 20, reference numeral 3001 denotes a substrate; and 3004, a conductive thin film made of a metal oxide formed by sputtering. This conductive thin film 3004 has an H-shaped pattern, as shown in FIG. 20. An electron-emitting portion 3005 is formed by performing electrification processing (referred to as forming processing to be described later) with respect to the conductive thin film 3004. An interval L in FIG. 20 is set to 0.5 to 1 mm, and a width W is set to 0.1 mm. The electron-emitting portion 3005 is shown in a rectangular shape at the center of the conductive thin film 3004 for the sake of illustrative convenience. However, this does not exactly show the actual position and shape of the electron-emitting portion.
In the above surface-conduction emission type electron-emitting devices by M. Hartwell et al. and the like, typically the electron-emitting portion 3005 is formed by performing electrification processing called forming processing for the conductive thin film 3004 such that electrons are emitted from the portion 3005. In forming processing, a constant DC voltage or a DC voltage which increases at a very low rate of, e.g., 1 V/min is applied across the conductive thin film 3004 to partially destroy or deform the conductive thin film 3004, thereby forming the electron-emitting portion 3005 with an electrically high resistance. Note that the destroyed or deformed part of the conductive thin film 3004 has a fissure. Upon application of an appropriate voltage to the conductive thin film 3004 after forming processing, electrons are emitted near the fissure.
Known examples of the FE type electron-emitting devices are described in W. P. Dyke and W. W. Dolan, “Field emission”, Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, “Physical properties of thin-film field emission cathodes with molybdenium cones”, J. Appl. Phys., 47, 5248 (1976).
FIG. 21 is a sectional view showing the device by C. A. Spindt et al. described above as a typical example of the FE type device structure. In FIG. 21, reference numeral 3010 denotes a substrate; 3011, an emitter wiring made of a conductive material; 3012, an emitter cone; 3013, an insulating layer; and 3014, a gate electrode. In this device, a voltage is applied between the emitter cone 3012 and gate electrode 3014 to emit electrons from the distal end portion of the emitter cone 3012.
As another FE type device structure, there is an example in which an emitter and gate electrode are arranged on a substrate to be almost parallel to the surface of the substrate, in addition to the multilayered structure of FIG. 21.
A known example of the MIM type electron-emitting devices is described in C. A. Mead, “Operation of Tunnel-Emission Devices”, J. Appl. Phys., 32,646 (1961).
FIG. 22 shows a typical example of the MIM type device structure. FIG. 22 is a sectional view of the MIM type electron-emitting device. In FIG. 22, reference numeral 3020 denotes a substrate; 3021, a lower electrode made of a metal; 3022, a thin insulating layer having a thickness of about 100 Å; and 3023, an upper electrode made of a metal and having a thickness of about 80 to 300 Å. In the MIM type electron-emitting device, an appropriate voltage is applied between the upper and lower electrodes 3023 and 3021 to emit electrons from the surface of the upper electrode 3023.
Since the above-described cold cathodes can emit electrons at a temperature lower than that for thermionic cathodes, they do not require any heater. The cold cathode has a structure simpler than that of the thermionic cathode and can shrink in feature size. Even if a large number of devices are arranged on a substrate at a high density, problems such as heat fusion of the substrate hardly arise. In addition, the response speed of the cold cathode is high, while the response speed of the thermionic cathode is low because thermionic cathode operates upon heating by a heater.
For this reason, applications of the cold cathodes have enthusiastically been studied.
Of cold cathodes, the surface-conduction emission type electron-emitting devices have a simple structure and can be easily manufactured, and thus many devices can be formed on a wide area. As disclosed in Japanese Patent Laid-Open No. 64-31332 filed by the present applicant, a method of arranging and driving a lot of devices has been studied.
Regarding applications of the surface-conduction emission type electron-emitting devices to, e.g., image forming apparatuses such as an image display apparatus (display) and image recording apparatus, charge beam sources, and the like have been studied.
Particularly as an application to image display apparatuses, as disclosed in the U.S. Pat. No. 5,066,883 and Japanese Patent Laid-Open Nos. 2-257551 and 4-28137 filed by the present applicant, an image display apparatus using a combination of an surface-conduction emission type electron-emitting device and a fluorescent substance which emits light upon collision with electrons has been studied. This type of image display apparatus using a combination of the surface-conduction emission type electron-emitting device and fluorescent substance is expected to exhibit more excellent characteristics than other conventional image display apparatuses. For example, compared with recent popular liquid crystal display apparatuses, the above display apparatus is superior in that it does not require any backlight because it is of a emissive type and that it has a wide view angle.
A method of driving a plurality of FE type electron-emitting devices arranged side by side is disclosed in, e.g., U.S. Pat. No. 4,904,895 filed by the present applicant. As a known example of an application of FE type electron-emitting devices to an image display apparatus is a flat panel display reported by R. Meyer et al. [R. Meyer: “Recent Development on Microtips Display at LETI”, Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6-9 (1991)].
An example of an application of a larger number of MIM type electron-emitting devices arranged side by side to an image display apparatus is disclosed in Japanese Patent Laid-Open No. 3-55738 filed by the present applicant.
Of these image forming apparatuses using electron-emitting devices, a flat panel display is space-saving and lightweight, and thus receives a great deal of attention as a substitute for an image display apparatus of a cathode ray tube type.
There is proposed a flat panel display in which an electron source obtained by arranging electron-emitting devices in a matrix is housed in an airtight container. This airtight container is constituted such that a face plate having fluorescent substances and a rear plate having the electron source are made to face each other and sealed. The interior of the airtight container is kept at a vacuum of about 10−6 Torr. As the display area of the image display apparatus increases, demand is arising for a means for preventing deformation or destruction of the rear plate and face plate caused by the difference between inner and outer pressures of the airtight container. Thus, a structure support (to be referred to as a spacer or rib) made of a relatively thin glass plate to stand the atmospheric pressure is conventionally interposed between the rear plate and face plate.
A method of manufacturing a spacer to be interposed between a pair of substrates constituting an image forming apparatus is disclosed in U.S. Pat. No. 4,923,421, U.S. Pat. No. 5,063,327, U.S. Pat. No. 5,205,770, U.S. Pat. No. 5,232,549, U.S. Pat. No. 5,486,126, U.S. Pat. No. 5,509,840, and U.S. Pat. No. 5,721,050, EP-A-0725416, EP-A-0725417, EP-A-0725418, EP-A-0725419, and the like.
However, the image forming apparatus and flat panel display using the above-described spacer suffer the following problems.
First, when some of electrons emitted by an electron-emitting device near the spacer collide against the spacer, or ions produced owing the effect of emitted electrons are attached to the spacer, the spacer may be charged. The orbits of electrons emitted by the electron-emitting device are deflected by charge-up of the spacer. As a result, the electrons reach positions different from correct positions on the fluorescent substances of the face plate to display a distorted image near the spacer.
Second, since a high voltage Va of several hundred V or more (e.g., a high electric field of 1 kV/mm or more) is applied between the rear plate and face plate in order to accelerate electrons emitted by the electron-emitting device. This may cause surface discharge on the spacer surface. If the spacer is charged in the above manner, discharge may be induced.
To solve these problems, there is proposed a method of flowing a small current through the spacer to remove charges (Japanese Patent Laid-Open Nos. 57-118355 and 61-124031). According to this method, a high-resistance film is formed on the surface of an insulating spacer substrate to flow a small current through the spacer surface. The high-resistance film used here is made of tin oxide, a mixed crystal of tin oxide and indium oxide, or a metal.
However, when the electron-emitting duty is high, image distortion cannot be satisfactorily reduced depending on the type of image only by the method of removing charges using the high-resistance film. This problem arises when the high-resistance film, and upper and lower substrates, i.e., a face plate (to be referred to as an FP) and rear plate (to be referred to as an RP) are not sufficiently electrically connected, and charges concentrate around the connected portions.
To solve this problem, as shown in FIG. 23, films (electrodes) 25 lower in resistance than a high-resistance film 22 are formed on the side surface of an insulating spacer substrate 21 and its end surfaces in contact with a face plate 17 and rear plate 11. The low-resistance films (electrodes) 25 can ensure electrical contact between the upper and lower substrates 17 and 11 and high-resistance film 22. FIG. 23 shows the low-resistance films (electrodes) 25 formed on the end surfaces in contact with the face plate 17 and rear plate 11, sand the side surface in contact with these end surfaces. FIG. 23 is a sectional view showing the spacer when a section perpendicular to the rear plate plane is taken along a spacer-including plane.
If Va is set low without forming any high-resistance film 22, or the shape of the side surface of the insulating spacer substrate 21 is controlled, the first and second problems may be solved even in a spacer whose insulator is exposed in vacuum. In this case, however, when the potential of the end surface of the insulating spacer substrate 21 is varied, the orbits of emitted electrons may also vary. To prevent this, as shown in FIG. 27, even if the insulating spacer is interposed between the face plate and rear plate, the electrode (low-resistance film) 25 must be formed on at least one end surface of the spacer.
FIG. 24 is a schematic sectional view taken along the line A—A when the spacer substrate 21 in FIG. 23 is flat (plate). FIG. 25 is a schematic enlarged view showing an RP-side end portion B of the spacer circled in FIGS. 23 and 27. In FIG. 25, for descriptive convenience, no high-resistance film is formed on the surface of the spacer substrate 21. FIG. 26 is a perspective view schematically showing the spacer substrate 21 when the spacer substrate 21 is flat (plate). FIG. 31 is a perspective view when the spacer substrate 21 is columnar. When the spacer substrate is columnar, an end surface diameter R corresponds to a length L and thickness D of the flat spacer substrate.
The present invention discriminates the term “spacer” from the term “spacer substrate”. The “spacer substrate” has any film (e.g., the high-resistance film 22 or low-resistance film 25) on the surface, as shown in FIG. 23. On the other hand, the “spacer” generally means a member interposed between the face plate 17 and rear plate 11 so as to support them, and has at least the spacer substrate and low-resistance film (electrode).
A method of forming a metal film or high-conductivity material film on the end surface of a spacer is disclosed in Japanese Patent Laid-Open No. 8-180821, U.S. Pat. No. 5,561,343, U.S. Pat. No. 5,614,781, U.S. Pat. No. 5,675,212, U.S. Pat. No. 5,746,635, U.S. Pat. No. 5,742,117, U.S. Pat. No. 5,777,432, WO 94/18694A, WO 96/30926A, WO 98/02899A, WO 98/03986A, WO 98/28774A, and the like.
These references disclose various methods such as sputtering, resistance heating evaporation, coating, dipping, and printing as the method of forming a metal film or high-conductivity material film on the end surface of a spacer.
Of these formation methods, a method (liquid phase formation method) such as coating, dipping, or printing of coating a spacer with a liquid and sintering the spacer can preferably easily form the low-resistance film (electrode) 25 at low cost.
However, if the low-resistance film (electrode) 25 is formed on the spacer substrate 21 simply using the liquid phase formation method, the following problems may occur.
By the liquid phase formation method, the formation state of the low-resistance film (electrode) 25 greatly depends on the surface shape of the spacer substrate 21.
Particularly when the spacer substrate 21 has an edge of an almost right angle, as shown in FIGS. 26 and 31, the low-resistance film (electrode) 25 cannot be satisfactorily formed at the edge. More specifically, the low-resistance film (electrode) 25 may become thin at the edge during film formation to expose part of the high-resistance film or the insulating spacer substrate 21. As a result, electron orbits near the connected portions between the spacer, RP, and FP may shift from desired orbits.